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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,657 records · Page 92
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Near-infrared and blue cooperative Yb3+ luminescence in Lu3Sc2Ga3O12 nano-garnets

Highlights: • Lu{sub 3}Sc{sub 2}Ga{sub 3}O{sub 12} nano-garnets doped with Yb{sup 3+} ions were synthesized by sol-gel method. • Concentration dependent structural and luminescence properties were investigated. • Variation of blue cooperative luminescence as a function of laser pump power has been studied. • Mechanism of blue cooperative luminescence of Yb{sup 3+} ions in these nano-garnets has been described. - Abstract: Nanocrystalline Lu{sub 3}Sc{sub 2}Ga{sub 3}O{sub 12} garnet powders doped with different concentrations of trivalent ytterbium (Yb{sup 3+}) ions have been prepared by sol-gel method. These garnet nano-powders have been characterized for their structural and luminescence properties using X-ray diffraction, diffuse reflectance and luminescence spectra. Near-infrared and blue cooperative upconversion luminescence spectra and decay curves of Yb{sup 3+} ions in these nano-garnets have been measured as a function of the optically active ion concentration. Quenching of near-infrared and cooperative luminescence intensity beyond 5.0 mol% of Yb{sup 3+} ion concentration have been observed. Variation of blue cooperative luminescence intensity with pump power has also been discussed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Crystal structure, Raman spectra, infrared spectra and microwave dielectric properties of Li2Mg3Ti1-X(Mg1/3Ta2/3)XO6 (0 ≤ x ≤ 0.2) solid solution ceramics

Highlights: • Microwave properties of ε{sub r} = 14.70, Q·f = 200,600 GHz and τ{sub f} = −11.83 ppm/°C were obtained. • A small amount of (Mg{sub 1/3}Ta{sub 2/3}){sup 4+} can obviously improve properties. • Rietveld refinement and spectra were used to analyze properties. • Relationships between intrinsic parameters and properties were established. - Abstract: Li{sub 2}Mg{sub 3}Ti{sub 1-X}(Mg{sub 1/3}Ta{sub 2/3}){sub X}O{sub 6} (x = 0.00, 0.05, 0.10, 0.15, 0.20) solid solution ceramics were prepared by the conventional solid-state reaction method. Rietveld refinements, Raman spectra, infrared spectra, polarizability and bond energy were used to research the changes of microwave dielectric properties of these solid solutions. With the increase of x values from 0 to 0.2, dielectric constants (ε{sub r}) gradually increased, which could be explained by the variations of the polarizability, infrared resonant modes and Raman vibration modes. The variations of the quality factor (Q·f) could be explained by the changes of the bond valences. The temperature coefficient of resonant frequency (τ{sub ƒ}) correlated with the bond energy of TiO bonds. A small quantity of (Mg{sub 1/3}Ta{sub 2/3}){sup 4+} can obviously improve the microwave dielectric properties. Excellent microwave dielectric properties of ε{sub r} = 14.70, Q·f = 200,600 GHz and τ{sub f} = −11.83 ppm/°C were obtained for Li{sub 2}Mg{sub 3}Ti{sub 0.95}(Mg{sub 1/3}Ta{sub 2/3}){sub 0.05}O{sub 6} ceramic sintered at 1550 °C.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Surface normal emissivity measurements in uncoated and oxidized B-coated plasma facing component materials in the long wavelength infrared spectrum

Accurate surface emissivity measurements are integral to the fidelity of the infrared thermography diagnostic evaluation in fusion reactors. The emissivities of ATJ TM graphite, Sigrafine® (R6510) graphite, and TZM alloy were measured as a function of temperature in accordance with the Contact Thermometer Method in the ASTM E1933 standard. Samples were heated resistively in a high-vacuum chamber, and the surface temperature was monitored using a surface thermocouple and a Telops long-wavelength infrared camera. The surfaces of Sigrafine® graphite and TZM alloy samples were coated with 10 nm and 20 nm layers of oxidized boron and were also measured in the uncoated condition. The thickness was assessed from measurement by a quartz crystal microbalance (QCM). ATJ TM graphite was uncoated. The measured emissivity of ATJ TM Graphite ranged from 0.82 to 0.83, uncoated Sigrafine® graphite ranged from 0.76 to 0.98, and uncoated TZM alloy ranged from 0.11 to 0.13. Oxidized boron coatings increased the emissivity of the TZM alloy to 0.18–0.23 but decreased the emissivity of Sigrafine® graphite to 0.56–0.66 for a 10 nm coating and to 0.65–0.74 for a 20 nm coating. In contrast to the expected blackbody radiance, the emissivity of uncoated Sigrafine® graphite and TZM alloy did not monotonically increase with temperature from 100 to 500 ℃.

Emissivity↗
DOE OSTI2022

Photothermal Atomic Force Microscopy Coupled with Infrared Spectroscopy (AFM-IR) Analysis of High Extinction Coefficient Materials: A Case Study with Silica and Silicate Glasses

Photothermal atomic force microscopy coupled with infrared spectroscopy (AFM-IR) brings significant value as a spatially resolved surface analysis technique for disordered oxide materials such as glasses, but additional development and fundamental understanding of governing principles is needed to interpret AFM-IR spectra, since the existing theory described for organic materials does not work for materials with high extinction coefficients for infrared (IR) absorption. This paper describes theoretical calculation of a transient temperature profile inside the IR-absorbing material considering IR refraction at the interface as well as IR adsorption and heat transfer inside the sample. Furthermore, this calculation explains the differences in peak positions and amplitudes of AFM-IR spectra from those of specular reflectance and extinction coefficient spectra. It also addresses the information depth of the AFM-IR characterization of bulk materials. AFM-IR applied to silica and silicate glass surfaces has demonstrated novel capability of characterizing subsurface structural changes and surface heterogeneity due to mechanical stresses from physical contacts, as well as chemical alterations manifested in surface layers through aqueous corrosion.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2022

Infrared Spectroscopy of Li + Solvation in EmimBF 4 and in Propylene Carbonate: Ab Initio Molecular Dynamics and Experiment

Infrared (IR) spectra of solutions of the lithium salt LiBF 4 in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF 4 ) and in the organic solvent propylene carbonate (PC) are studied via infrared spectroscopy and ab initio molecular dynamics (AIMD) simulations. The measurements show that the major effects of LiBF 4 in both solutions, compared to their neat counterparts, are the appearance of a new broad band in the 300-450 cm -1 frequency region and a broadening of the IR structure in the 900-1200 cm -1 region with the development of a new peak at 980 cm -1 . Computational analysis indicates that hindered translational motions of Li + in its solvation cage are mainly responsible for the former, while the latter is due to Li + -induced structural changes and accompanying vibrational frequency shifts of constituent ions and molecules of the solutions. In addition, molecular motions in these and lower frequency regions are generally correlated, disclosing the collective nature of the vibrational dynamics, which involve multiple ions/molecules. Herein, detailed analysis of these features via AIMD simulations of the spectrum and its components arising from auto- and cross-correlations of motions of constituent molecular species, combined with generalized normal modes of the solutions and normal modes of small Li + -containing clusters, is presented. Furthermore, other minor spectral changes caused by the lithium salt as well as the interaction-induced effect on IR spectra are also discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

In Situ Attenuated Total Reflectance Infrared Spectroelectrochemistry (ATR-IR-SEC) for the Characterization of Molecular Redox Processes on Surface-Proximal Doped Silicon ATR Crystal Working Electrodes

In situ mid-infrared spectroscopy is a powerful technique for understanding the mechanism of CO 2 reduction (CO 2 R) catalysts because it enables the direct detection of catalytic intermediates and products. Moreover, spectroelectrochemistry (SEC), the coupling of spectroscopy with electrochemistry, allows spectroscopic changes to be correlated with applied potentials to reveal potential-dependent intermediates that are often relevant to photoelectrochemical reactions. Hybrid photoelectrodes, comprised of a narrow bandgap semiconductor, like silicon (Si), with a covalently-linked molecular catalyst, are a promising platform for sunlight-driven catalysis, but characterization of the catalytic mechanism(s) is challenging under photoelectrochemical conditions, particularly when the catalyst is present in monolayer or less concentrations. Here, we have developed a new strategy to use multiple-reflection attenuated total reflectance IR spectroscopy (ATR-IR) coupled with electrochemistry to characterize catalysts directly integrated with a semiconductor surface under applied potential. We show that by surface-proximal n-type or p-type doping of the top ~100-200 nm of the crystal surface, Si ATR crystals can be used simultaneously as the internal reflection element and semiconductor working electrode for ATR-IR-SEC measurements. The surface-proximal doping strategy yields a quasi-equipotential surface with excellent infrared transparency that would have been compromised by free carrier absorption if the crystal was uniformly doped. This approach permits the catalytically-active functionalized surface to be directly probed without modification and overcomes signal-to-noise limitations of other strategies that use separately deposited working electrodes on Si ATR crystals. Proof-of-concept ATR-IR-SEC spectra were collected during the reduction and oxidation of monolayers of Re- and Ru-based transition metal carbonyl complexes, respectively, verifying the viability of the technique to probe redox processes associated with CO 2 R catalysts on Si electrode surfaces with high sensitivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Infrared Imaging Using Thermally Stable HgTe/CdS Nanocrystals

Transferring nanocrystals (NCs) from the laboratory environment toward practical applications has raised new challenges. HgTe appears as the most spectrally tunable infrared colloidal platform. Its low-temperature synthesis reduces the growth energy cost yet also favors sintering. Once coupled to a read-out circuit, the Joule effect aggregates the particles, leading to a poorly defined optical edge and large dark current. Here, in this study, we demonstrate that CdS shells bring the expected thermal stability (no redshift upon annealing, reduced tendency to form amalgams, and preservation of photoconduction after an atomic layer deposition process). The electronic structure of these confined particles is unveiled using k.p self-consistent simulations showing a significant exciton binding energy of ~ 200 meV. After shelling, the material displays a p-type behavior that favors the generation of photoconductive gain. The latter is then used to increase the external quantum efficiency of an infrared imager, which now reaches 40% while presenting long-term stability.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Evolution of Active Sites in Pt-Based Nanoalloy Catalysts for the Oxidation of Carbonaceous Species by Combined in Situ Infrared Spectroscopy and Total X-ray Scattering

Here, we present results from combined in situ infrared spectroscopy and total X-ray scattering studies on the evolution of catalytically active sites in exemplary binary and ternary Pt-based nanoalloys during a sequence of CO oxidation–reactivation–CO oxidation reactions. We find that when within a particular compositional range, the fresh nanoalloys may exhibit high catalytic activity for low-temperature CO oxidation. Using surface-specific atomic pair distribution functions (PDFs) extracted from the in situ total X-ray scattering data, we find that, regardless of their chemical composition and initial catalytic activity, the fresh nanoalloys suffer a significant surface structural disorder during CO oxidation. Upon reactivation in oxygen atmosphere, the surface of used nanoalloy catalysts both partially oxidizes and orders. Remarkably, it largely retains its structural state when the nanoalloys are reused as CO oxidation catalysts. The seemingly inverse structural changes of studied nanoalloy catalysts occurring under CO oxidation and reactivation conditions affect the active sites on their surface significantly. In particular, through different mechanisms, both appear to reduce the CO binding strength to the nanoalloy’s surface and thus increase the catalytic stability of the nanoalloys. The findings provide clues for further optimization of nanoalloy catalysts for the oxidation of carbonaceous species through optimizing their composition, activation, and reactivation. Besides, the findings demonstrate the usefulness of combined in situ infrared spectroscopy and total X-ray scattering coupled to surface-specific atomic PDF analysis to the ongoing effort to produce advanced catalysts for environmentally and technologically important applications.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Accelerated Discovery of Cost-Effective Photoabsorber Materials for Near-Infrared (λ = 1600 nm) Photodetector Applications

Current infrared sensing devices are based on costly materials with relatively few viable alternatives known. To identify promising candidate materials for infrared photodetection, we have developed a high-throughput screening methodology based on high-accuracy r 2 SCAN and HSE calculations in density functional theory. Using this method, we identify ten already synthesized materials between the inverse perovskite family, the barium silver pnictide family, the alkaline pnictide family, and ZnSnAs 2 as top candidates. Among these, ZnSnAs 2 emerges as the most promising candidate due to its experimentally verified band gap of 0.74 eV at 0 K and its cost-effective synthesis through Bridgman growth. BaAgP also shows potential with an HSE-calculated band gap of 0.64 eV, although further experimental validation is required. Lastly, we discover an additional material, Ca 3 BiP, which has not been previously synthesized, but exhibits a promising optical spectra and a band gap of 0.56 eV. The method applied in this work is sufficiently general to screen wider bandgap materials in high-throughput and now extended to narrow-band gap materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Quantitative Mid-infrared Photoluminescence Characterization of Black Phosphorus–Arsenic Alloys

Black phosphorus (bP) is a promising material for mid-infrared (mid-IR) optoelectronic applications, exhibiting high performance light emission and detection. Alloying bP with arsenic extends its operation toward longer wavelengths from 3.7 μm (bP) to 5 μm (bP 3 As 7 ), which is of great practical interest. Quantitative optical characterizations are performed to establish black phosphorus-arsenic (bPAs) alloys optoelectronic quality. Anisotropic optical constants (refractive index, extinction coefficient, and absorption coefficient) of bPAs alloys from near-infrared to mid-IR (0.2-0.9 eV) are extracted with reflection measurements, which helps optical device design. Quantitative photoluminescence (PL) of bPAs alloys with different As concentrations are measured from room temperature to 77 K. PL quantum yield measurements reveal a 2 orders of magnitude decrease in radiative efficiency with increasing As concentration. An optical cavity is designed for bP 3 As 7 , which allows for up to an order of magnitude enhancement in the quantum yield due to the Purcell effect. In conclusion, our comprehensive optical characterization provides the foundation for high performance mid-IR optical device design using bPAs alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Solution-Synthesized High-Mobility Tellurium Nanoflakes for Short-Wave Infrared Photodetectors

Two-dimensional (2D) materials, particularly black phosphorus (bP), have demonstrated themselves to be excellent candidates for high-performance infrared photodetectors and transistors. However, high-quality bP can be obtained only via mechanical exfoliation from high-temperature- and high-pressure-grown bulk crystals and degrades rapidly when exposed to ambient conditions. In this paper, we report solution-synthesized and air-stable quasi-2D tellurium (Te) nanoflakes for short-wave infrared (SWIR) photodetectors. We perform comprehensive optical characterization via polarization-resolved transmission and reflection measurements and report the absorbance and complex refractive index of Te crystals. It is found that this material is an indirect semiconductor with a band gap of 0.31 eV. From temperature-dependent electrical measurements, we confirm this band-gap value and find that 12 nm thick Te nanoflakes show high hole mobilities of 450 and 1430 cm 2 V –1 s –1 at 300 and 77 K, respectively. Finally, we demonstrate that despite its indirect band gap, Te can be utilized for high-performance SWIR photodetectors by employing optical cavity substrates consisting of Au/Al 2 O 3 to dramatically increase the absorption in the semiconductor. By changing the thickness of the Al 2 O 3 cavity, the peak responsivity of Te photoconductors can be tuned from 1.4 μm (13 A/W) to 2.4 μm (8 A/W) with a cutoff wavelength of 3.4 μm, fully capturing the SWIR band. An optimized room-temperature specific detectivity (D*) of 2 × 10 9 cm Hz 1/2 W –1 is obtained at a wavelength of 1.7 μm.

2D materials↗
DOE OSTI2020

All-Epitaxial Integration of Long-Wavelength Infrared Plasmonic Materials and Detectors for Enhanced Responsivity

Here, infrared detectors using monolithically integrated doped semiconductor “designer metals” are proposed and experimentally demonstrated. We leverage the “designer metal” groundplanes to form resonant cavities with enhanced absorption tuned across the long-wave infrared (LWIR). Detectors are designed with two target absorption enhancement wavelengths: 8 and 10 μm. The core of our detectors are quantum-engineered LWIR type-II superlattice p-i-n detectors with total thicknesses of only 1.42 and 1.80 μm for the 8 and 10 μm absorption enhancement devices, respectively. Our 8 and 10 μm structures show peak external quantum efficiencies of 45 and 27%, which are 4.5× and 2.7× enhanced, respectively, compared to control structures. We demonstrate the clear advantages of this detector architecture, both in terms of ease of growth/fabrication and enhanced device performance. The proposed architecture is absorber- and device-structure agnostic, much thinner than state-of-the-art LWIR T2SLs, and offers the opportunity for the integration of low dark current LWIR detector architectures for significant enhancement of IR detectivity.

42 ENGINEERING↗
DOE OSTI2019

Hybrid Metasurface-Based Mid-Infrared Biosensor for Simultaneous Quantification and Identification of Monolayer Protein

Metasurfaces emerge as a promising photonic platform for biosensing because they offer strong optical confinement and tunable optical resonances. Here, we show that metasurface-based biosensors consisting of gold nanoantenna arrays loaded with graphene and working in the mid-infrared (mid-IR) spectral range can achieve simultaneous high-sensitivity and high-specificity detection of biomolecules. Strong light–molecule interactions in deeply subwavelength optical spots created by the biosensors allow us to determine the concentration of protein molecules via spectral shifts of the metasurface resonance. A combination of passive and active tuning of the metasurface sensors allows us to spectrally overlap the metasurface resonance and the protein vibrational modes, so that protein molecules can be identified via their characteristic mid-IR “fingerprints”. As a result, the high sensitivity and specificity of the metasurface sensors enable us to determine the secondary structure of protein immunoglobulin (IgG) molecules 4 orders of magnitude more sensitive than attenuated total reflection Fourier transform infrared spectroscopy.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Depth Thermography: Noninvasive 3D Temperature Profiling Using Infrared Thermal Emission

Here, we introduce a technique based on infrared thermal emission, termed depth thermography, that can remotely measure the temperature distribution beneath the surface of certain objects. Depth thermography utilizes the thermal-emission spectrum in the semitransparent spectral region of the target object to extract its temperature as a function of depth, in contrast with conventional thermography, which uses the spectrally integrated thermally emitted power to measure the surface temperature. Coupled with two-dimensional imaging, for example using an infrared hyperspectral camera or scanning a single-pixel spectrometer, this technique can yield volumetric temperature distributions. Furthermore, we carried out a proof-of-concept experiment on an asymmetrically heated fused-silica window, extracting the temperature distribution throughout the sample. Depth thermography may enable noncontact volumetric temperature measurements of microscopic objects such as multilayer electronic devices or macroscopic volumes of liquids and gases, as well as simultaneous all-optical measurements of optical and thermal properties of materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Surface-Enhanced Infrared Absorption: Pushing the Frontier for On-Chip Gas Sensing

Surface-enhanced infrared absorption (SEIRA) is capable of identifying molecular fingerprints by resonant detection of infrared vibrational modes through the coupling with plasmonic modes of metallic nanostructures. However, SEIRA for on-chip gas sensing is still not very successful due to the intrinsically weak light-matter interaction between photons and gas molecules and the technical challenges in accumulating sufficient gas species in the vicinity of the spatially localized enhanced electric field, namely, the “hot-spots”, generated through plasmonics. In this paper, we present a suspended silicon nitride (Si 3 N 4 ) nanomembrane device by integrating plasmonic nanopatch gold antennas with metal–organic framework (MOF), which can largely adsorb carbon dioxide (CO 2 ) through its nanoporous structure. Unlike conventional SEIRA sensing relying on highly localized hot-spots of plasmonic nanoantennas or nanoparticles, the device reported in this article engineered the coupled surface plasmon polaritons in the metal–Si 3 N 4 and metal–MOF interfaces to achieve strong optical field enhancement across the entire MOF film. Lastly, we successfully demonstrated on-chip gas sensing of CO 2 with more than 1800× enhancement factors by combining the concentration effect from the 2.7 μm MOF thin film and the optical field enhancement of the plasmonic nanopatch antennas.

47 OTHER INSTRUMENTATION↗
DOE OSTI2014

Alumina(0001)/Water Interface: Structural Properties and Infrared Spectra from First-Principles Molecular Dynamics Simulations

We investigated the atomic structure and infrared spectra of the alumina(0001)/water interface, using first-principles molecular dynamics simulations based on density functional theory within the generalized gradient approximation. The computed structural properties of the interface are in good agreement with the results of synchrotron X-ray experiments. In this work detailed analyses of the computed infrared spectra revealed two types of water molecules at the hydrophilic oxide/water interface: molecules participating in strong “ice-like” hydrogen bonding with the oxide surface and molecules involved in weaker “liquid-like” hydrogen bonding. Our results provide a molecular interpretation of the “ice-like” and “liquid-like” bands observed in sum-frequency vibrational spectroscopy experiments and underscore the significance of strong hydrogen-bonding interactions in determining the orientation of interfacial water molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

High-sensitivity infrared vibrational nanospectroscopy in water

Mid-infrared vibrational spectroscopy is a universal label-free tool for identifying molecular compounds in chemical and biological samples on the basis of their ‘fingerprint’ vibrational absorption lines. Vibrational spectroscopy with nanometer spatial resolution can reveal the chemical composition of samples at the nanoscale, and several scanning-probe techniques have been developed to address this need. It is also highly desirable to study biological and chemical samples in their native aqueous environments rather than in air. In aqueous environments, however, the sensitivity of the current vibrational nanospectroscopy techniques deteriorates dramatically . Here, we report the first mid-infrared nanospectroscopy technique that retains nanoscale sensitivity and spatial resolution when the sample and the scanning probe are completely immersed in water. This method overcomes challenges including water absorption and scattering and the mechanical damping of cantilever vibrations. We further demonstrate spectroscopy and imaging of 20- to 50-nm-thick polymer samples with a 25-nm spatial resolution in the biologically relevant Amide I and II spectral regions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Reactions between atomic chlorine and pyridine in solid para-hydrogen: Infrared spectrum of the 1-chloropyridinyl (C{sub 5}H{sub 5}N-Cl) radical

With infrared absorption spectra we investigated the reaction between Cl atom and pyridine (C{sub 5}H{sub 5}N) in a para-hydrogen (p-H{sub 2}) matrix. Pyridine and Cl{sub 2} were co-deposited with p-H{sub 2} at 3.2 K; a planar C{sub 5}H{sub 5}N-Cl{sub 2} complex was identified from the observed infrared spectrum of the Cl{sub 2}/C{sub 5}H{sub 5}N/p-H{sub 2} matrix. Upon irradiation at 365 nm to generate Cl atom in situ and annealing at 5.1 K for 3 min to induce secondary reaction, the 1-chloropyridinyl radical (C{sub 5}H{sub 5}N-Cl) was identified as the major product of the reaction Cl + C{sub 5}H{sub 5}N in solid p-H{sub 2}; absorption lines at 3075.9, 1449.7, 1200.6, 1148.8, 1069.3, 1017.4, 742.9, and 688.7 cm{sup -1} were observed. The assignments are based on comparison of observed vibrational wavenumbers and relative IR intensities with those predicted using the B3PW91/6-311++G(2d, 2p) method. The observation of the preferential addition of Cl to the N-site of pyridine to form C{sub 5}H{sub 5}N-Cl radical but not 2-, 3-, or 4-chloropyridine (ClC{sub 5}H{sub 5}N) radicals is consistent with the reported theoretical prediction that formation of the former proceeds via a barrierless path.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗